The present invention relates to balancing the load in communication between a central communication center (satellite or other communication center) and a plurality of users that share resources (bandwidth and/or power limited channels). More specifically, the load is balanced by allocating resources between different users based on signal degradation as determined for the different users.
The communication center allocates resources and transmits the signals to the plurality of users. Examples of communications centers include terrestrial cellular Hub stations, gateways that transmit to users through a satellite, and processing satellites. In applications using a satellite channel and for the purpose of this invention, the central communication center can be considered to include both the gateway and the satellite.
Degradation of a user signal may be mitigated by allocating more channel resources to that user. However, in a resource limited channel user requirements including quality of service (QOS) and throughput can not always be satisfied. In a typical communication channel the performance of each user signal is variable from factors including link characteristics, requirements, propagation effects, signal distortion, losses and interference. This invention can be used to optimize the allocation of resources to ensure equitable sharing of the channel by a plurality of users.
Satellite channels are resource limited and user performance is likely to be variable over the satellite beam. In satellite channels operating at very high frequencies (>10 GHz) users located in rain cells can experience very large rain fades, while other users in the same beam may be operating in clear sky. One common method for overcoming these rain fades is to dynamically reduce the data rates of faded users. However, this is an unacceptable solution if the data rate reductions are too severe. For example, if the link to a user is for a satellite telephone service, a severe reduction in the data rate will make the sound quality go down and/or cause some words to be lost in the communication. In an internet data delivery service, severe data rate reductions will result in unacceptable delays to the user.
The satellite beam is usually much larger than the rain cells and therefore, it can be assumed that only a small percentage of users are heavily faded at any particular time. Since there are only a small number of faded users, this invention allows each unfaded user to give up a small portion of the satellite resource, to the faded users, in order to balance the load between the different users and provide a consistent QOS for all users.
Various U.S. patents relating to communications techniques will be discussed.
Gurantz U.S. Pat. No. 4,837,786 discloses a quadrature phase shift transmission arrangement where a satellite communicates with ground stations using two separate transmission channels (one with a higher data rate and one with a lower data rate) on one frequency. The lower data rate channel is used for a ground station when rain fade occurs.
Acampora U.S. Pat. No. 4,495,619 discloses a TDMA system where time slots are allocated on the basis of carrier-to-noise ratio to achieve low rain outage.
Acampora U.S. Pat. No. 4,301,533 discloses a rain margin increasing technique where time slots are dedicated to a beam experiencing a rain fade or similar condition.
Langseth U.S. Pat. No. 4,287,598 shows a satellite arrangement where alternate path (to alternate ground station then via ground cable route) is used for ground stations experiencing rain fade.
Jasper U.S. Pat. No. 5,533,004 is a TDMA system where a modulation technique is selected with blocks formatted into an equal number of information bits.
Hester U.S. Pat. No. 5,355,374 communication system where bandwidth is reallocated as needed.
Other communication system and technique U.S. patents include:
Acampora4,730,305Strodtbeck5,864,547Palmer5,625,640Rosen5,625,624
Communication satellites usually provide service to a large number of customers distributed over a wide geographic area. Since at any given time rainfall rates will vary within this area, the downlink rain fades to each customer will also have a significant variation.
Although many of the prior designs have been generally useful, they have often been subject to one or more of several disadvantages. A severe data rate reduction to a particular user may effectively disable the communications link to that user.
Some techniques rely on excess capacity which is used for particular links that are experiencing difficult conditions, such as heavy rain. However, the excess capacity is wasted when none of the users is experiencing difficult conditions. Alternately, the excess capacity might be utilized to improve signal quality to a higher level than needed. In either case, the excess capacity requires more bandwidth than otherwise.
Some techniques change the signal constellations according to the transmission conditions. However, the capability to operate using different signal constellations substantially increases the complexity of the transceiver systems both at the user end and at the satellite or other communication center.